Update BFS impls

I've subtly been implementing breadth-first traversals in graphs
incorrectly. The change is subtle, but updating `seen` needs to happen
immediately after queuing an item.

The results will remain the same, but the runtimes will differ dramatically. I
didn't notice this until I attempted to complete LeetCode's "count islands"
challenge, and LeetCode rejected my solution because it could not finish before
timing out. After looking at other candidates' solutions and comparing them to
mine, I couldn't see any difference... except for this subtle difference.

This SO answer provides a helpful explanation:
https://stackoverflow.com/questions/45623722/marking-node-as-visited-on-bfs-when-dequeuing

The take-away lesson here is to always call `seen.add(..)` immediately after
enqueuing.

1 files changed, 1 insertions, 1 deletions

diff --git a/scratch/facebook/count-islands.py b/scratch/facebook/count-islands.py
index a007684bca23..b876319b2f7a 100644
--- a/scratch/facebook/count-islands.py
+++ b/scratch/facebook/count-islands.py
@@ -8,6 +8,7 @@ def maybe_queue(row, col, game, q, seen):
     if row >= 0 and row < len(game) and col >= 0 and col < len(game[0]):
         if game[row][col] == 'L' and (row, col) not in seen:
             q.append((row, col))
+            seen.add((row, col))
 
 def visit_island(row, col, game, seen):
     """
@@ -18,7 +19,6 @@ def visit_island(row, col, game, seen):
     q.append((row, col))
     while q:
         row, col = q.popleft()
-        seen.add((row, col))
         maybe_queue(row - 1, col, game, q, seen) # UP
         maybe_queue(row + 1, col, game, q, seen) # DOWN
         maybe_queue(row, col - 1, game, q, seen) # LEFT